This invention relates to digital optical receiver circuits, and more particularly to such circuits which operate over a wide input dynamic range (e.g. from 3 microwatts/cm.sup.2 to 1.5 watts/cm.sup.2, or 56.9 db) without distorting its output signal. In general, digital optical receiver circuits are designed to operate over a very narrow input dynamic range e.g., 10 db, or with fixed input power level. Such design is commonly used because most optical communications links communicate between fixed distances. Consequently such circuits have a relatively high input sensitivity to permit operation on a low power budget. However, such circuits also tend to overload at input power only slightly greater than the minimal required input because of the large amplification used. An example of such circuit is shown in FIG. 1. Such circuits when overloaded result in seriously distorted output signals due to amplifier saturation. In some cases, such circuits will latch into one logic state for an unpredictable time period, or give an unpredictable, unintelligible output when the specified input levels are exceeded. However, in the case of weapons training systems utilizing laser transmitters mounted on the weapon to simulate the firing of the weapon, such weapon laser may fire at a target over a wide range of distances from point blank range to a range beyond the useful limits of the weapon sight. Under such circumstances, the receiver circuit is inherently subject to a wide dynamic range of incident power levels while a large amount of information is being transmitted by the laser beam. Thus any distortion of the received signal, particularly if it becomes latched-up, will likely cause incorrect decoding or a failure to decode the signal.
Consequently an object of the present invention is a digital optical receiver circuit adapted to operate over a wide input dynamic range without distorting its output signal. Another object of the present invention is such receiver circuit which can operate in full sunlight with a low false output rate due to sun-induced noise. Still another object of the present invention is such receiver circuit which operates from a single power supply with low power consumption. Still another object of the present invention is such receiver circuit which provides a high capacitance output drive capability but permits multiple units to be connected in parallel without affecting its operating characteristics.